W126 300SD 
Located at the front of the engine, the actuator is very easy to access and test. The connector is indicated by the smaller red circle. 

W126 300SDL (OM603) / W124 300D (OM603)

Located under the intake manifold, on the drivers side of the engine. Connector is on the driver's side fender. Both are circled in red

 

 

W126 420SEL / 560SEL

Located at the front passenger side of the engine. Connector is mounted to the base of the coolant bottle bracket.

R107 560SL

Located at front passenger side of engine. Connector is mounted adjacent to the passenger headlight.

W123  300TD / 300D / 240D

Actuator is located at the front of the engine, driver's side. Connector is located on the driver's side fender. Photos are from PeachParts forum member neumann's 1985 300TD. Locations are probably applicable to the 300D and 240D as well.

R129 500SL 

The R129 500SL's actuator is located in the front of the engine just next to the power steering reservoir. 

W124 300D 2.5 Turbo (OM602)

The acuator is attached to the intake manifold. The actuator in this picture has been painted black. The connector is located on the driver's side fender.